Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:
3GPPthird generation partnership projectCBcodebookCoMPcoordinated multi-point transmission/receptionCQIchannel quality indicatorCRCcyclic redundancy checkCSIchannel state information (for example, CQI, PMI, and RI)CSI-RS channel state information reference symbolsCWcodewordDLdownlinkeNBbase station of an EUTRAN/LTE systemEUTRANevolved UTRAN (also referred to as LTE or 3.9 G)LTElong term evolutionITUinternational telecommunication unionITU-RITU radiocommunication sectorMCSmodulation and coding schemeMIMOmultiple-input-multiple-outputMU-MIMOmulti-user multiple input multiple outputNrnumber of receive antennasNtnumber of transmit antennasOFDMAorthogonal frequency division multiple accessPDCCHphysical downlink control channelPRBphysical resource blockPMIprecoding matrix indicatorPUCCHphysical uplink control channelPUSCHphysical uplink shared channelRIrank indicatorRSreference symbolsSU-MIMOsingle-user multiple-input-multiple-outputTBStransport block sizeTXtransmitUEuser equipmentULuplinkUTRAuniversal mobile telecommunication system terrestrial radio accessUTRANUTRA networkXPcross polarized/polarization
In the communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE, E-UTRA or 3.9G), the LTE Rel. 8 is completed, the LTE Rel. 9 is being standardized, and the LTE Rel. 10 is currently under development within the 3GPP. In the downlink (DL), LTE Rel. 10 will support 8-Tx DL SU-MIMO with up to eight spatial layers (streams) as well as enhanced DL MU-MIMO transmission.
FIG. 1 reproduces FIG. 4.1 of 3GPP TS 36.300, V8.6.0 (2008-09), and shows the overall architecture of the E-UTRAN system. The EUTRAN system comprises eNBs, providing the EUTRA user plane and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a Mobility Management Entity and to a Serving Gateway. The S1 interface supports a many to many relationship between Mobility Management entities/Serving Gateways and eNBs.
Of particular interest herein are the further releases of 3GPP LTE targeted towards future IMT-A systems, referred to herein for convenience simply as LTE-Advanced (LTE-A). LTE-A is directed toward extending and optimizing the 3GPP LTE Release 8 radio access technologies to provide higher data rates at very low cost. LTE-A will most likely be part of LTE Release 10. LTE-A is expected to use a mix of local area and wide area optimization techniques to fulfill the ITU-R requirements for IMT-Advanced while keeping the backward compatibility with LTE Release 8. In order to meet the peak spectral efficiency requirements (up to 30 bit/s/Hz) support of up to 8 TX antennas in DL will be standardized in LTE Release 10, enabling DL spatial multiplexing transmission with up to 8 spatial layers. Both 8-TX DL MIMO and enhanced MU-MIMO are now agreed to be in the Release 10 for enhanced DL MIMO transmission.
What is needed in LTE Release 10 is a codebook design for 8 transmit (TX) antennas. In RAN1#59 it has been agreed to extend the Release 8 implicit feedback framework to LTE Release 10. This is based on a modular design (or multi-granular), combining two feedback components from distinct codebooks: one feedback component which represents the long-term (for example, wideband) channel properties while the other feedback component targets short term (for example, frequency selective) channel properties.
Comparing to LTE Release 8/9, two new flavours of DL MIMO are considered for LTE Release 10:                The optimization of MU-MIMO operation, which benefits from a new reference symbol design package in terms of precoded UE specific reference symbols (sometimes referred to as UE-RS or demodulation reference symbols DRS or DM-RS) and periodic channel state information reference symbols (CSI-RS).        Extension to up to 8-layer DL MIMO operation (i.e. spatial multiplexing with up to eight spatial streams).        
These enhancements will be supported by a new UE feedback mode for channel state information (CSI) and channel quality indication (CQI), following the implicit feedback principles in LTE Release 8. Accurate CSI feedback is important, especially for MU-MIMO. Moreover, signalling aspects and codebook sizes take on a greater importance when considering the extension to 8-layer SU-MIMO operation.
Codebook (CB) entries in LTE Release 8 have been defined for up to rank 4 transmission and follow several design constraints like constant modulus, finite alphabet and nested property. Operating such CBs is rather straightforward: based on the estimated channel over common reference symbols (CRS), the UE determines its preferred transmission rank for this channel and based on this selects the codewords based on a selection criteria like for example throughput maximization. For brevity term these codebook definitions and operations as traditional CB or single codebook operation.
There have been several contributions in LTE Release 10 for codebooks based on the traditional CB design. For example, document R1-101462 by Motorola proposes several CBs of 4-5 bits size, with codewords allowing operation in both Uniform Linear Arrays (ULA) scenarios and also in cross-polarized (XP) scenarios. Other proposals for traditional CB designs were submitted in RAN1#60.
During RAN1#60, it has been agreed to extend the Release 8 feedback mode consisting of implicit signalling of PMI/RI/CQI (see for example document R1-101683). The agreed modular (multi-granular) structure of the CB consist of two precoding matrices, one targeting the wideband and/or long term channel properties the other targeting the frequency-selective and/or short term channel properties. Proposals in this regard have been made in RAN #59, for example by Huawei (document R1-10025 entitled: Downlink 8Tx codebook considerations) and by Ericsson (document R1-100051, entitled: A flexible feedback concept).
These teachings are directed toward multi-granular structured CBs, and explore certain considerations for designing such multi-granular CBs for best use in systems supporting high and/or multiple transmission ranks for SW and MU-MIMO such as LTE Release 10.